Clean hydrogen has attracted attention in recent years as an energy source substituting for fossil fuels owing to the warming of global climate due to CO.sub.2.
In view of this situation, attention has been directed to hyrogen absorbing alloys as novel functional materials. These alloys include two-component alloys, such as ZrMn.sub.2, TiMn.sub.2, TiCr.sub.2, Mg.sub.2 Ni, TiFe and LaNi.sub.5, which are safe and easy to handle and which have reversible reactivity with hydrogen at pressure levels of 0.1 to 1 MPa, and quasi two-component alloys resembling such alloys and comprising more components.
Hydrogen absorbing alloys perform outstanding functions in converting energy and purifying hydrogen and are regarded as functional materials which are indispensable to the efficient use of hydrogen energy, for example, for use in storing or transporting hydrogen or in heat pumps utilizing the reaction heat thereof.
The equilibrium reaction between the hydrogen absorbing alloy and hydrogen is evaluated with reference to a hydrogen pressure-composition (hydrogen content) isotherm (P-C-T isotherm).
For example, FIG. 3 shows a P-C-T isotherm of LaNi.sub.5 which is a hydrogen absorbing alloy at a temperature of 298K. With reference to the diagram wherein the hydrogen content is plotted as abscissa vs. the equilibrium hydrogen pressure as ordinate, the P-C-T isotherm has a hydrogen solid solution region (.alpha. phase region) and a metal hydride region (.beta. phase region) which are great in gradient, and a generally horizontal plateau region between the two regions.
Especially, the width of the plateau region corresponds to the content of hydrogen which is effectively movable in the system. The greater the value of effective hydrogen content, the smaller is the amount of alloy needed and the system can be more efficient and smaller in size.
The plateau region is a region where three phases are conjointly present. According to Gibbs' phase rule, this region is a horizontal portion exhibiting a constant pressure independently of the composition of the hydrogen absorbing alloy. In the actual metal-hydrogen reaction, however, this portion has a positive inclination due to the heterogeneity of the alloy from the viewpoint of metal engineering.
For example when the hydrogen absorbing alloy is applied to heat accumulation systems, a very great disadvantage will result with respect to efficiency if the plateau region has a great inclination since the hydrogen content due to a given pressure difference is then small. For this reason, efforts are made to render the plateau region horizontal as by making the alloy homogeneous by heat treatment.
Accordingly, modeling of P-C-T isotherms, if possible, is very significant because the models are not only useful for designing various application systems wherein hydrogen absorbing alloys are used, simulated operation of such systems and evaluation thereof but also permit accumulation and use of equilibrium characteristics data of various alloys to provide guides in designing and developing more efficient alloys.
Heretofore known as modeling P-C-T isotherms are formulation based on a regular solution model in Pd-H system ("A Theroretical Formula for the Solubility of Hydrogen in Palladium," R. Soc. London, Ser A.161 (1937), pp. 525-545) and formulation based on adsorption isothermal formula of the Langmuir type in LaNi.sub.4.79 Al.sub.0.21 --H sytstem ("A NUMERICAL EXPRESSION FOR THE P-C-T PROPERTIES OF METAL HYDRIDES," Journal of the Less-Common Metals, 130(1987), pp. 365-370).
However, the former has the drawback of being unable to fully express the inclination of the plateau although capable of accurately exressing the hydrogen solid solution region (.alpha. phase region) and metal hydride region (.beta. phase region).
On the other hand, the latter is capable of expressing the overall P-C-T isotherm region almost satisfactorily, but the parameters included in the expression have no physicochemical meaning, the P-C-T isotherm being merely modeled by numerical analysis, so that it is not reasonable to use the parameter obtained by modeling for the evaluation of the equilibrium characteristics of the hydrogen absorbing alloy.